Read from standard input an arithmetic expression in postfix notation (reverse Polish notation), made up of natural numbers and the operators +, -, *, /. Tokens (numbers and operators) are separated by single spaces.

Compute the value of the expression using a stack of numbers implemented on an array with index top. The algorithm is:

• when you encounter a number, push it onto the stack;
• when you encounter an operator, pop two values from the stack (b then a), compute a op b, and push the result back onto the stack.

At the end, exactly one value remains on the stack: the result.

Do not use STL containers.